Toward Modeling the Structure of Electrolytes at Charged Mineral Interfaces Using Classical Density Functional Theory

Area of Science:

• Physical Chemistry
• Colloid Science
• Materials Science

Background:

• The behavior of water and ions at charged interfaces is critical for understanding colloidal suspensions and particulate gels.
• Existing models often struggle to capture the complex interplay of forces governing these systems.

Purpose of the Study:

• To develop a comprehensive density functional model for the free energy of water and ions in electric double layers.
• To accurately predict the structure and forces at charged mineral-water interfaces.

Main Methods:

• Assembled a density functional incorporating fundamental measure theory (finite particle size), statistical association theory (hydrogen-bonding), high-temperature expansion (dispersion forces), functionalized mean-spherical approximation (electrostatic correlations), and Poisson equation (Coulomb forces).
• Applied the model to planar geometries near graphene and mica surfaces, explicitly including mica's outer oxygen layer for hydrogen bonding.
• Calculated disjoining pressure between like-charged surfaces.

Main Results:

• The model reproduces correlated structures, including counterion and co-ion layering at charged surfaces.
• It accurately captures ion and surface solvation through combined short-range and long-range interactions.
• Predicted pressure oscillations during ion/water expulsion and strong attractive stresses at narrow gaps due to overscreening and out-of-plane structuring.

Conclusions:

• The developed density functional provides a robust framework for studying interfacial phenomena in charged systems.
• The model's ability to predict pressure oscillations and attractive stresses offers new insights into colloidal stability and gel formation.
• Explicitly modeling surface structure and water-surface interactions enhances predictive accuracy for realistic mineral interfaces.